1. Field of the Invention
This apparatus relates to a vessel for the refining of molten aluminum. More particularly, it relates to a protective lining for such a vessel.
2. Description of the Prior Art
In aluminum refining vessels, the refining chamber is frequently an externally heated cast iron tub. If the tub walls were bare, the turbulent molten aluminum present therein during refining operations would dissolve the cast iron at a very rapid rate. This would result in a very short tub life, e.g., no more than a few days for a cast iron wall 11/2 inches thick. Such dissolving of the cast iron would also result in an unacceptable iron contamination of the aluminum. To slow down this unacceptable wash-out process, the cast iron tub is completely lined with refractory plates and shapes. In the area of the cast iron tub wall that is externally heated, the lining is composed of graphite. Because graphite has a much higher thermal conductivity than any other material that is resistant to attack by molten aluminum, it is the only suitable material for such use. If the tube lining were made of the next best material from a thermal conductivity standpoint, e.g., silicon carbide, the extra temperature drop throught the lining, because of its lower thermal conductivity, would necessarily result in a higher, excessive tub wall temperature and a consequent rapid failure of the cast iron because of cracking, bulging and the like.
Such a refractory lining does not serve to keep molten aluminum from contacting the tub wall. It would be very difficult, and certainly impractical, to make a lining that was completely liquid tight. Not only would this be difficult to accomplish, but it would also be undesirable, again for thermal conductivity reasons. Molten aluminum that occupies the space between the lining and the tub wall provides an excellent thermal conduction path between the two parts. If this space were only gas filled, the tube wall would have to be much hotter in order to transfer the required amount of heat to the interior of the refining vessel. This, in turn, would lead to an early failure of the cast iron tub.
If the molten aluminum that penetrates the space between the refractory lining and the tub wall is static, it will dissolve iron from the cast iron tub until it becomes saturated, this being about 2 to 3% iron at normal operating temperatures. Under the worst circumstances (from the tub wall viewpoint), the molten aluminum will react with enough iron to form an iron aluminum containing 42% iron. This level of iron consumption represents only an insignificant loss of iron from the cast iron tub wall. Significant losses of iron occur, on the other hand, when molten aluminum is allowed to circulate into and out of this space. If there are openings in the lining, such circulation will occur, driven by thermal density gradients, composition density gradients (aluminum with dissolved iron being more dense than pure aluminum), and to a very great extent, during refining operations, by the fluid forces created by the spinning nozzle employed in such operations. Such circulating currents have been known to wash out, i.e. dissolve a hole through, a 11/2 inch thick gray iron tub wall in a few weeks. This type of circulation generally occurs when the molten aluminum from the refining space within a vessel enters the space between the lining and the tub wall through a small hole or a slot between two parts of the refractory lining.
A part of the problem of tub wash-out is caused by the loss of graphite due to oxidation. When the refining system is at idle and is not well inerted on the inside of the vessel, the portion of the graphite lining plates above the molten aluminum level will be lost as a result of oxidation. This can be controlled by careful sealing of the refining space, but, in practice, this is not commonly done in many aluminum refining shops. Once, some part of a graphite plate has been oxidized away down to below the operating level of molten aluminum in the vessel, the side wall of the cast iron tub will no longer be protected at that point. While that particular part of the tub wall may be coated with enough dross to prevent actual contact between the cast iron of the tub and the molten aluminum, the molten aluminum nevertheless has a large entry point for passage into the space behind the lining. If there also is an exit point due to openings between lining plates and shapes, particularly one near the bottom of the refining vessel, then rapid circulation of molten aluminum behind the lining can occur, resulting in the undesired, rapid wash-out of the cast iron tub wall.
Oxidation of the graphite lining above the idle level can be effectively eliminated by covering this portion of the graphite plate with a non-oxidizable material that is not attached by molten aluminum. Silicon nitride bonded silicon carbide is a good material for this purpose. A skirt of this material can be placed so as to rest on top of the graphite plate and be clamped to the cast iron tub so that it will maintain its position on top of the graphite plate and not slide off into the vessel. This clamping also serves to hold the graphite plates down and prevents said plates from floating upward when the vessel assembly is filled with molten aluminum. The upper end of the graphite plate is thus held or effectively clamped against the cast iron tub wall. The silicon carbide skirt that rests on top of the graphite plate extends downward over the inner surface of said graphite plate past the upper operating molten aluminum level and to below the lower idle molten aluminum level, so as to afford protection for the graphite against oxidation in the refining space above the level of molten aluminum in the vessel.
In order to eliminate most of the channels for flow of molten aluminum into and out of the space between the graphite lining and the tub wall, the bottom, sides and at least one end of the vessel are desirably lined with single pieces of graphite with no through openings. The side plates and the end plate are joined to the bottom plate, typically by known tongue and groove joints. When the lining is installed in the cast iron tub in this manner, the various pieces are fitted close together and against the tub walls, and any gaps between the joined plates are filled with cement.
When the refining vessel is heated to operating temperature, the tub expands more than the lining because of its higher thermal expansion coefficient. Under this circumstance, the tub on longer holds the pieces of the lining in close contact with one another. Since the graphite side and end plates are clamped to the walls of the tub by the refractory skirts as indicated above, these graphite plates are actually pulled apart at the upper end thereof. The lower ends of the graphite plates, however, are held in contact with one another by their tongue and groove joints with the bottom plate. This movement creates openings between the side plates and end plate at their upper ends, thus providing a channel for the flow of molten aluminum between the refining space within the vessel and the space between the graphite lining and the cast iron tub. A tongue and groove joint cannot be used between the back graphite plate and the side graphite plates because such a joint would restrain the necessary outward motion of either the side or the end plate during heat-up. Such restraint would result in either fracture of the tongue and groove joint or in the dislodging of the refractory skirt or the breaking of the graphite plates. It is highly desirable, however, that a means be found to create a joint not subject to the opening of a channel for molten aluminum flow upon the necessary movement of the graphite plates upon heating the refining vessel to operating temperature.
It is an object of the invention, therefore, to provide an improved joint between the graphite side plates and the end plates of an aluminum refining vessel.
It is a further object of the invention to provide a joint between said graphite side plates and the end plate that will allow relative motion as required during heat-up, while still maintaining an effective barrier to the flow of molten aluminum through the joint.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being pointed out in the appended claims.